The present invention relates generally to wireless networks and more particularly to a system and method for accurately locating a client among access points in a wireless network.
Wireless networks in accordance with the IEEE 802.11 and 802.3 standards typically make no use of access point location information. For example, one pulse measure technique, referred to as time domain reflectometry, for use in cable and connection testing in association with Gigabit Ethernet physical interface cards has been documented. However, time domain reflectometry is merely used to measure cable length, or to measure the cable length to a cable break. No attempt is made to locate access points or to use the length information to distribute a synchronized clock through the network.
Time difference of arrival techniques are generally known for locating a radio in a wireless network. Time difference of arrival techniques require at least three receive points in the network to locate a radio. Moreover, to accurately locate a radio in a network using a time difference of arrival technique, two additional requirements must also be met. The first requirement is that the relative locations of the access points must known, and the second requirement is that a common, precise time reference must be shared by the access points. These two additional requirements follow for a wireless local area network.
Once the relative locations are known and a time reference is shared, the following procedure allows for locating a radio or a client. First, the client for which the location is desired transmits a packet. Each access point receives and records the packet, time stamping the packet using the time reference. Each recorded packet is then post processed to identify precisely when the packet arrived with respect to the start of the record. The offset of the arrival, plus the time stamp, is when the packet arrived at a respective access point. The arrival times for each access point are then compared with the access point locations, and the location of the client that transmitted the packet is calculated using known algorithms relating to a circle.
Unfortunately, wireless networks in accordance with the IEEE 802.11 and 802.3 standards typically lack sufficient clock correlation in the switch and the access points to allow for accurately locating a client. For example, network time protocols or simple network time protocols generally allow for a variation of 1 microsecond (μsec) in timing. 1 μsec translates to propagation distance of approximately 983 feet (ft.), or the variation or degree of accuracy one can expect in the location of a client should such a protocol be used to locate a client. Thus, 1 μsec of variation in the synchronization of network entities is too long a period of time to allow for accurately locating a client in a wireless network.
One approach to locating a radio in a wireless network is to make use of received signal strength indicator information. Generally, switches and access points have and use received signal strength indicator signals. Received signal strength indicator signals are voltage signals indicative of received signal strength or power, and are commonly used to control gain. Received signal strength indicator information is also generally available over the network.
However, the use of received signal strength indicator information does not allow for accurately locating a client in a wireless network. For example, the output power of client radios is known to vary substantially from one radio to the next. Variations of 3 decibels (dB), or half the output power, are not uncommon. Thus, two radios in the same location having different output powers results in two different received signal strength indicator voltages. Consequently, two different locations for each client are arrived at, even though the radios are located in the same place. Therefore, due to radio output power variations, received signal strength indicator information does not allow for accurately locating a client in a wireless network.
Thus, there exists a need for distributing a precise time reference to network entities in a wireless network. Moreover, there exists a need for a system and method for locating a client in a wireless network.